Assuming the intent is to kill, then Zabuza and Haku have the advantage with their Hiddn mist and silent killing techniques, not to mention it would be two vs one with both having powerful long range ninjutsus and masters of their chosen weapons. If zabuza puts up a mist and Haku uses the ice mirrors then its basically over for him.
Ive tried seeing how Itachi could fight his way out of this with his genjutsus, but going against 2 people is bad for a genjutsu user unless the person can use a wide AoE genjutsu, which itachi doesn't have. His fire jutsus are hopelessely outclassed and so are his water jutsus, and since itachi wont know of Haku's KKG and cant really see it with his sharingan nor can he copy it, he will have a hard time getting accustomed to his fighting style. His only hope is his taijutsu and speed which is considerably better then theirs especially Haku who makes up for it with his speed and ninjutsu though I doubt he would be able to overwhelm a kenjutsu master and his proficient senbon user at the same time, even with a sharingan.
Susano, followed by Yasaka Magatama machine gun everywhere until Zabuza and Haku are eliminated.
Alive itachi dies after one yasaka magatama
Susano'o and other MS jutsu are restricted. I'm still pretty sure Itachi would function fine in the mist though, as Kakashi did against Edo Zabuza. Itachi's faster and more skilled, so I'd bet on him beating them down old-school, not unlike what Sharingan Kakashi did to Zabuza in their part one rematch (despite Kakashi being less developed than Itachi in key areas.)
Plus a major counter to mist IMO is to use large katons to remove the moisture in the air.
Katons would be countered by suitons, making even more mist.
Sick Itachi definitely is not above Kakashi in terms of speed and taijutsu and his eyesight isn't the best.
I think Haku and Zabuza take this extremely high diff. Itachi would be looking out for Zabuza while Haku keeps throwing senbons at him. HE can't defend from 2 directions.
While Itachi can use ninjutsu quickly, Haku himself has powerful ice and water jutsus that he can use with only one hand and relatively few seals, making him one of the many people who could easily keep up with him in ninjutsu.
(BTW I finally got my new internet today! xD)
Sick Itachi definitely is not above Kakashi in terms of speed and taijutsu and his eyesight isn't the best.
Sick Itachi is factually far above Wave Arc Kakashi in speed. The databook proves as much. Itachi's better at taijutsu too.
And his hand-speed makes him much better with weapons. He's just vastly superior, physically.